The Marlowe Papers is a novel in verse by the poet Ros Barber, who wrote the book as part of a PhD and has also published academic papers on Christopher Marlowe. Its main fictional contention is that the playwright was not killed "in a tavern brawl" in Deptford, but spirited away by colleagues in the secret service (with which he was undoubtedly involved), lived sometimes abroad and sometimes in England, took the name William Shakespeare and wrote the plays we (or most of us) always thought were produced by the Swan of Avon.

Even a thumbnail sketch is enough to show that Barber's book is a rara avis. From one point of view it appears enormously ambitious, narratively fascinating, ingenious and impossible to ignore. From another it seems overstretched, ludicrously conjectural, pointlessly elaborate and interesting only as an oddity.

It would be unreasonable simply to plump for one or other of these views, because the book is so manifestly a labour of love, as well as a labour. Its success or failure depends on assessing two distinct endeavours that are linked in a very unusual way. Barber has set herself a peculiar problem: if the poetry is good but the research is bad or implausible, the creative element is made to seem pointless. And if the poetry is bad but the research good, the reader will wonder why on earth the content wasn't delivered in a more orthodox manner.

As it turns out, even this kind of judgment is difficult to make, because Barber has had mixed success with both aspects of the book. To start with the poetry: most of it is written in a relaxed version of iambic pentameter, punctuated here and there by more formal episodes, usually sonnets. For most of it her ear remains pretty reliable, and the metre keeps the story rolling along; when problems occur, they are generally to do with the difficulties of reconciling the proper concentration of poetry with a narrative demand for information, which produces a dispiriting effect of prose: "His conscience seemed troubled all the same. No, truly, / I noticed that he couldn't stop your name / peppering every sentence."

A more frequent and serious difficulty has to do with tone. Barber says that she has tried "to avoid cod Elizabethan" and also to "strike a balance between authenticity and readability" – both sensible aims, but neither prosecuted with enough flair to prevent the poem from stumbling quite often into Elizabethan-lite: "Still, the past draws me like a jug of beer / back to the moments when my star was high. / The greater the heights, the more extreme the fall. / And in those glorious nights, the splintered how / of waking up breached and broken on my now." There's no denying that these lines echo and therefore respect ideas we find in Marlowe (and indeed Shakespeare), or that they have the vague flavour of antiquated speech (partly because of the way they play with the iambic pentameter). But this flavour is dissipated by the puzzling "splintered how" and the seeming modernity of the climatic "my now". In seeking to avoid certain things and to "strike a balance", Barber has ended up writing a poem that for much of its duration sounds like a patchwork. There is individuality of a kind, but not a very plausible kind.

And this question of plausibility is crucial, because readers are likely to feel that if they can't trust the poetry, they're less willing to trust the story. As things turn out, it's not easy to see how much Barber trusts it herself. To start with, it's comparatively plain sailing: we have a versified account of Marlowe's early life and writing, his friendships in the secret service and elsewhere, a credible suggestion of the spy-infested state and the jealousy-infested writing community, all of which end in the pub in Deptford – if we believe the usual version of events. For the afterlife, Barber acknowledges that she has built her story of Marlowe's European travels, his later playwriting, his occasional furtive visits to England, his relationship with the Earl of Southampton and his bisexuality (cue "Shakespeare's" sonnets) on "a sturdy skeleton of research that was largely the work of others".

Like most stories about contested authorship, especially those involving Shakespeare, this skeleton engrosses some and seems ridiculous to many. As far as Barber's own book is concerned, the point is not so much whether they are worth the time of day as whether she adds to them in a significant way or makes them more credible. Sometimes it's hard to say (because the fancy-loving form she's chosen has a distinctly destabilising effect on "hard facts") and at others all too easy (because the poetry doesn't create a sufficiently dynamic self-contained universe to be persuasive).

So where does this leave us, and Barber herself? In her acknowledgments she tells us that the Shakespeare scholar Jonathan Bate "provided me with my lightbulb moment when he said, of the 'crazy' idea that Marlowe faked his death and escaped into exile, 'I do think there is a really good novel in here.'" Quite possibly there is. But in choosing to write it as a verse novel, Barber has produced something in which the constituent parts work against one another more often than they work together. In the end, while respecting the scale of the endeavour, it's difficult not to feel that for long periods they threaten to cancel one another out.